Non-platy minerals such as calcium carbonate, calcium sulfate, barium sulfate and the like which have been treated to produce particles having, in the aggregate, relatively high surface areas are known to be useful as fillers for various thermosetting and thermoplastic resinous compositions. Such fillers are primarily extenders rather than reinforcing fillers and can be used alone or mixed with other fillers, including reinforcing fillers such as talc, glass fibers and the like.
U.S. Pat. No. 4,257,817, issued March 24, 1981 to Mathur et al, discloses inorganic fillers having reduced binder demand for use in thermosetting and thermoplastic resin formulations. Mathur et al's fillers are prepared by a method which involves subjecting the filler to the high intensity rotary impact agitation of a blade rotating at a peripheral speed of from about 20 to 60 meters per second. The patentees make passing reference to:
"(p)ast attempts at reducing the binder demand of inorganic fillers [which] have included such approaches as alteration of the particle size distribution of the filler to minimize its void volume, compaction techniques and surface coating of the filler", PA1 "A series of sheet molding compound (SMC) formulations based on (1) standard calcium carbonate fillers, (2) developmental `dry` ground calcium carbonate fillers, (3) blends of standard and developmental `dry ` ground calcium carbonate fillers, and (4) calcium carbonate fillers produced by `specific classification` production techniques were compounded into paste and SMC prodccts of various types by making changes in the components, conditions and temperature-moisture influences and tabulating the observed changes resulting from varying the herein described variables."
col 1, lines 30-37. No further details appear, however, regarding such approaches, nor are the particle size characteristics or particle size distributions of these reduced binder demand fillers given in the Mathur et al patent.
An article by Fekete et al entitled "A Study of the Influence of Specific Variables on Viscosity Relationships and Surface Properties of Sheet Molding Compound [SMC] Formulations Based on Special Calcium Carbonate Fillers" from the proceedings of the 32nd Annual Technical Conference, 1977, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., Section 7-B, pp. 1-17, reports that:
Page 1, "Summary". The Fekete et al article then mentions blends of "Camel Wite" and "Camel Wite `D`" with "Camel Tex" and "Camel Carb" calcium carbonate fillers, and "specifically classified" fillers, and discusses thickening rates in unsaturated polyester sheet molding resin systems containing such fillers. The filled resins whose viscosities were tested by Fekete et al had calcium carbonate: resin ratios of about 1.5:1, maximum. No particle size characteristics or particle size distributions are given for any of the fillers discussed in the Fekete et al article.
In an earlier article by Fekete referred to in the Fekete et al article: "Influence of Ground Limestone on the Surface Properties of SMC Compositions", proceedings of the 31st Annual Technical Conference, 1976, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., Section 7-A, pp. 1-18, particle size characteristics and particle size distributions are listed in Tables XXIV and XXV, p. 18, for "Camel Wite" and "Camel Tex", identified in both the Fekete and Fekete et al articles as "standard", i.e., wet ground, calcium carbonate fillers. And here too the filled resins evaluated by the author had calcium carbonate: resin ratios of about 1.5:1, maximum.